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A RFB with quinones as aromatic redox-active organic molecules instead of redox-active metals was developed with the main objective to reduce capital costs [129]. The used molecule in the anolyte was 9,10-anthraquinone-2,7-disulphonic acid (AQDS) dissolved in 1 M H2SO4. According to the authors, AQDS can be synthesized from cheap, conventional educts. Fig 10. Chemical structure of 9,10-anthraquinone-2,7-disulphonic acid. Reprinted with permission from [129]. In the positive half-cell the redox couple was Br2/Br- complementing the quinone/hydroquinone redox couple in the anolyte. For this type of battery a tremendous power density of 1 W cm-2 was reported at a relatively low cell voltage of 0.86 V [130,131]. The reached storage capacity retention was more than 99% per cycle (99.90% per cycle over 40 cycles). These losses are thought to originate from leakage of the anolyte, crossover of bromide and destruction of redox species [132]. The theoretical capacity of 35 Wh L-1 cannot be reached in the cell, only 23 Wh L-1 could be realized. A possible explanation was provided by Carney et al., who found intermolecular dimerization of AQDS at concentrations greater than 10 mM [133] . This behaviour is pH dependent and might reduce the electrons that can be carried per AQDS molecule from 2 to 1.5 in acidic media. A further advantage is that the chemical and electrochemical properties of the AQDS molecules can be engineered by adding functional groups. Addition of a hydroxyl group reduces the standard potential of the redox reaction, thereby increasing the open circuit potential UOCV. A CV of 1 mM solution of AQDS in 1 M sulphuric acid on a glassy carbon disk working electrode was conducted and a fast reversible two-electron two-proton redox reaction occurred. The peak separation was 34 mV and corresponds to a two-electron process. The kinetic reaction rate constant k0 was calculated to 7.25 x 10-3 cm s-1 which is greater than that found for other components used in for RFBs such as V3+/V2+ (compare table 1). The main advantages of the AQDS/Br system are low electrolyte costs and fast Page 36 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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